OCR A Level Chemistry A Notes — 6.1 Benzene and Aromatic Compounds

In electrophilic substitution, an electrophile replaces a hydrogen atom on an aromatic ring.

The existing group changes electron density around the ring. It can also change how readily the ring reacts. Predicting both the reaction rate and the favoured position helps plan a synthesis.

Electron-donating groups, such as –CH₃, –OH and –NH₂, increase electron density in the ring’s π system. They usually activate the ring toward electrophilic substitution and favour substitution at the 2 and 4 positions relative to the original group.

During bromination of methylbenzene, the methyl group directs the incoming bromine mainly to positions 2 and 4. The products include 2-bromotoluene and 4-bromotoluene, with bromine on the ring at those positions. Nitration follows the same positional pattern.

An electron-withdrawing group, such as –NO₂, removes electron density from the aromatic π system. The ring becomes less reactive toward electrophiles. A nitro group directs further substitution mainly to the 3 position relative to itself.

Bromination of nitrobenzene gives mainly 3-bromonitrobenzene. The nitro group both deactivates the ring and favours the meta position. State both effects when explaining the product distribution.

Identify the group already on the ring, decide whether it donates or withdraws electron density, then number possible substitution positions from that group. Donating groups in this lesson point to 2 and 4; the nitro group points to 3.

Directing effects indicate the major products rather than a guarantee that only one product forms. If two different directing groups are present, more detailed analysis may be needed to predict the dominant position.